Methods and apparatus for low-loss reconfigurable optical quadrature amplitude modulation (QAM) signal generation

ABSTRACT

In some embodiments, an apparatus includes a quadrature amplitude modulation (QAM) optical modulator which includes a first phase modulator (PM), a second PM, a tunable optical coupler (TOC), and an optical combiner (OC). The TOC is configured to split a light wave at an adjustable power splitting ratio to produce a first split light wave and a second split light wave. The first PM is configured to modulate the first split light wave in response to a first multi-level electrical signal to produce a first modulated light wave. The second PM is configured to modulate the second split light wave in response to a second multi-level electrical signal to produce a second modulated light wave. The OC is then configured to combine the first modulated light wave and the second modulated light wave to generate a QAM optical signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of U.S. patent application Ser. No.15/255,078, now U.S. Pat. No. 10,367,586, filed Sep. 1, 2016, andentitled “Methods and Apparatus for Low-Loss Reconfigurable OpticalQuadrature Amplitude Modulation (QAM) Signal Generation”, the disclosureof which is incorporated herein by reference in its entirety.

BACKGROUND

Some embodiments described herein relate generally to methods andapparatus for optical quadrature amplitude modulation (QAM) signalgeneration. In particular, but not by way of limitation, someembodiments described herein relate to methods and apparatus forlow-loss reconfigurable optical QAM signal generation.

With a growing demand of optical communication systems with high datarates capability, optical quadrature amplitude modulation (QAM) signalsare generated to provide high data-carrying capacity and high spectralefficiency. Higher-order QAM signals are currently generated by usingelectronic circuits to drive in-phase and quadrature (IQ) opticalmodulators. These approaches, however, present challenges such as theneed for expensive digital-to-analog converters (DAC), high modulationloss of a typical IQ modulator, inefficient use of the non-linear regionof the modulator transfer function, and a lack of reconfigurability ofthe QAM signals.

Accordingly, a need exists for improved and simplified methods andapparatus to achieve low-loss reconfigurable optical QAM signalgeneration.

SUMMARY

In some embodiments, an apparatus includes a quadrature amplitudemodulation (QAM) optical modulator, which includes a first phasemodulator, a second phase modulator, a tunable optical coupler, and anoptical combiner. Each of the first phase modulator and the second phasemodulator is operatively coupled to the tunable optical coupler and theoptical combiner. The tunable optical coupler is configured to split alight wave at an adjustable power splitting ratio to produce a firstsplit light wave and a second split light wave. The first phasemodulator is configured to modulate the first split light wave inresponse to a first multi-level electrical signal to produce a firstmodulated light wave. The second phase modulator is configured tomodulate the second split light wave in response to a second multi-levelelectrical signal to produce a second modulated light wave. The opticalcombiner is then configured to combine the first modulated light waveand the second modulated light wave to generate a QAM optical signal.

In some embodiments, an apparatus includes a quadrature amplitudemodulation (QAM) optical modulator, which includes an optical splitter,a first phase modulator, a second phase modulator, and an opticalcombiner. The optical splitter, having a first output and a secondoutput, is configured to receive an optical signal and split the opticalsignal according to a first power splitting ratio. The first phasemodulator, operatively coupled to the first output of the opticalsplitter, is configured to receive a first split optical signal from thefirst output and modulate the first split optical signal to produce afirst modulated optical signal such that the first modulated opticalsignal is represented within a constellation diagram as covering a firstplurality of constellation points on a circle. The second phasemodulator, operatively coupled to the second output of the opticalsplitter, is configured to receive a second split optical signal fromthe second output of the optical splitter and modulate the second splitoptical signal to produce a second modulated optical signal such thatthe second modulated optical signal is represented within theconstellation diagram as starting from a point on the circle andcovering a second plurality of constellation points. The first pluralityof constellation points and the second plurality of constellation pointsrepresent all constellation points of a QAM optical signal. The opticalcombiner, operatively coupled to the first phase modulator and thesecond phase modulator at a second power coupling ratio, is configuredto combine the first modulated optical signal and the second modulatedoptical signal to produce an output modulated QAM optical signal.

In some embodiments, an apparatus includes a parallel quadratureamplitude modulation (QAM) optical modulator, which includes a firsttunable optical coupler (TOC), a second TOC, a first phase modulator(PM), a second PM, a third PM, a first optical combiner (OC), and asecond optical combiner (OC). The first TOC, having a first output and asecond output, is configured to split an optical signal. The first PM isoperatively coupled to the first output of the first TOC and a firstinput of the first OC. The second TOC, having a first output and asecond output, is operatively coupled to the second output of the firstTOC. The second PM is operatively coupled to the first output of thesecond TOC and a first input of the second OC. The third PM isoperatively coupled to the second output of the second TOC and a secondinput of the second OC. An output of the second OC is coupled to asecond input of the first OC. The first OC is configured to output amodulated QAM optical signal.

In some embodiments, an apparatus includes a serial quadrature amplitudemodulation (QAM) optical modulator including a first phase modulator(PM), a first tunable optical coupler (TOC), a second PM, a second TOC,a third PM, a first optical combiner (OC), and a second optical combiner(OC). The first PM, operatively coupled to an input of the first TOC, isconfigured to receive an input optical signal. The first TOC has a firstoutput operatively coupled to the second PM and a second outputoperatively coupled to the first OC. The second PM is operativelycoupled to an input of the second TOC. The second TOC has a first outputoperatively coupled to the third PM and a second output operativelycoupled to the second OC. The third PM is operatively coupled to aninput of the second OC and an output of the second OC is coupled to afirst input of the first OC. The first OC is configured to output amodulated QAM optical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is a block diagram illustrating a quadrature amplitude modulation(QAM) optical modulator, according to an embodiment.

FIGS. 2A-2B are constellation diagrams of a 16-QAM optical signalgenerated by a QAM optical modulator, according to an embodiment.

FIG. 3 illustrates constellation diagrams of a 16-QAM optical signalbased on simulations of output generated by a QAM optical modulator,according to an embodiment.

FIG. 4 illustrates constellation diagrams of a 64-QAM optical signalbased on simulations of output generated by a QAM optical modulator,according to an embodiment.

FIG. 5 illustrates constellation diagrams of a 256-QAM optical signalbased on simulated output generated by a QAM optical modulator,according to an embodiment.

FIG. 6 is a block diagram illustrating a parallel optical signaltransmission apparatus configured to generate m-QAM optical signals,according to an embodiment.

FIG. 7 is a block diagram illustrating a serial optical signaltransmission apparatus configured to generate m-QAM optical signals,according to an embodiment.

FIG. 8 is a flow chart illustrating a method of generating a QAM opticalsignal with a QAM optical modulator having two phase modulators,according to an embodiment.

FIG. 9 is a flow chart illustrating a method of generating a QAM signalwith a parallel or serial QAM optical modulator having more than twophase modulators, according to an embodiment.

DETAILED DESCRIPTION

In some embodiments, an apparatus includes a quadrature amplitudemodulation (QAM) optical modulator which includes a first phasemodulator, a second phase modulator, a tunable optical coupler, and anoptical combiner. Each of the first phase modulator and the second phasemodulator is operatively coupled to the tunable optical coupler and theoptical combiner. The tunable optical coupler is configured to split alight wave at an adjustable power splitting ratio to produce a firstsplit light wave and a second split light wave. The first phasemodulator is configured to modulate the first split light wave inresponse to a first multi-level electrical signal to produce a firstmodulated light wave. The second phase modulator is configured tomodulate the second split light wave in response to a second multi-levelelectrical signal to produce a second modulated light wave. The opticalcombiner is then configured to combine the first modulated light waveand the second modulated light wave to generate a QAM optical signal.

In some embodiments, an apparatus includes a quadrature amplitudemodulation (QAM) optical modulator which includes an optical splitter, afirst phase modulator, a second phase modulator, and an opticalcombiner. The optical splitter, having a first output and a secondoutput, is configured to receive an optical signal and split the opticalsignal according to a first power splitting ratio. The first phasemodulator, operatively coupled to the first output of the opticalsplitter, is configured to receive a first split optical signal from thefirst output and modulate the first split optical signal to produce afirst modulated optical signal such that the first modulated opticalsignal is represented within a constellation diagram as covering a firstplurality of constellation points on a circle. The second phasemodulator, operatively coupled to the second output of the opticalsplitter, is configured to receive a second split optical signal fromthe second output of the optical splitter and modulate the second splitoptical signal to produce a second modulated optical signal such thatthe second modulated optical signal is represented within theconstellation diagram as starting from a point on the circle andcovering a second plurality of constellation points. The first pluralityof constellation points and the second plurality of constellation pointsrepresent all constellation points of a QAM optical signal. The opticalcombiner, operatively coupled to the first phase modulator and thesecond phase modulator at a second power coupling ratio, is configuredto combine the first modulated optical signal and the second modulatedoptical signal to produce an output modulated QAM optical signal.

In some embodiments, an apparatus includes a parallel quadratureamplitude modulation (QAM) optical modulator which includes a firsttunable optical coupler (TOC), a second TOC, a first phase modulator(PM), a second PM, a third PM, a first optical combiner (OC), and asecond optical combiner (OC). The first TOC, having a first output and asecond output, is configured to split an optical signal. The first PM isoperatively coupled to the first output of the first TOC and a firstinput of the first OC. The second TOC, having a first output and asecond output, is operatively coupled to the second output of the firstTOC. The second PM is operatively coupled to the first output of thesecond TOC and a first input of the second OC. The third PM isoperatively coupled to the second output of the second TOC and a secondinput of the second OC. An output of the second OC is coupled to asecond input of the first OC. The first OC is configured to output amodulated QAM optical signal.

In some embodiments, an apparatus includes a serial quadrature amplitudemodulation (QAM) optical modulator including a first phase modulator(PM), a first tunable optical coupler (TOC), a second PM, a second TOC,a third PM, a first optical combiner (OC), and a second optical combiner(OC). The first PM, operatively coupled to an input of the first TOC, isconfigured to receive an input optical signal. The first TOC has a firstoutput operatively coupled to the second PM and a second outputoperatively coupled to the first OC. The second PM is operativelycoupled to an input of the second TOC. The second TOC has a first outputoperatively coupled to the third PM and a second output operativelycoupled to the second OC. The third PM is operatively coupled to aninput of the second OC and an output of the second OC is coupled to afirst input of the first OC. The first OC is configured to output amodulated QAM optical signal.

In some embodiments, an apparatus includes a quadrature amplitudemodulation (QAM) optical modulator to generate a QAM optical signal. TheQAM optical modulator can be configured to include two phase modulators(PM), a tunable optical coupler (TOC), and an optical combiner (OC).Each of the two PMs can be operatively coupled to the TOC and the OC.When the QAM optical modulator is operatively coupled to an opticalsource that emits an optical signal, the TOC can split the opticalsignal, according to a power splitting ratio, to produce a first splitoptical signal and a second split optical signal. The power splittingratio can be fixed or dynamically adjusted during a design process, amanufacturing process, a reconfiguration process, a troubleshootprocess, or in operation of the QAM optical modulator. Each of the PMscan modulate the first split optical signal and the second split opticalsignal respectively to produce a first modulated optical signal and asecond modulated optical signal. The OC can then combine the firstmodulated optical signal and the second modulated optical signal,according to a power coupling ratio, to produce the QAM optical signal.Similarly, the power coupling ratio can be fixed or dynamically adjustedduring a design process, a manufacturing process, a reconfigurationprocess, a troubleshoot process, or in operation of the QAM opticalmodulator. Unlike a typical QAM in-phase/quadrature (I/Q) modulator,which performs intensity modulation, the QAM optical modulator describedherein can perform phase only modulation, in some embodiments. Thus, thetwo phase modulators of the QAM optical modulator cover constellationpoints on circles. Compared with the typical QAM I/Q modulator, the QAMoptical modulator described herein greatly reduces modulation loss andprovides more reconfigurability.

As used in this specification, the singular forms “a,” “an” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, the term “an optical modulator” is intended to mean asingle optical modulator or multiple optical modulators. For anotherexample, the term “a phase modulator” is intended to mean a single phasemodulator or multiple phase modulators.

FIG. 1 is a block diagram illustrating an optical signal transmissionapparatus, according to an embodiment. In some embodiments, the opticalsignal transmission apparatus 100 includes an optical source 102 and aquadrature amplitude modulation (QAM) optical modulator 101 operativelycoupled to the optical source 102. The optical source 102 can be, forexample, a laser diode emitting an optical signal (or light wave) 110 ina continuous waveform (CW). The optical source 102 outputs the opticalsignal 110 to the QAM optical modulator 101, which can modulate theoptical signal 110 to output a QAM optical signal 120 for transmissionin an optical communication system (not shown in FIG. 1). The QAMoptical signal 120 is modulated in a way that can be represented as oneof 2′ constellation points. Details of the representation of 2′constellation points are discussed below with regards to FIGS. 2A-2B.

In some embodiments, the QAM optical modulator 101 includes a firstphase modulator (PM1) 106, a second phase modulator (PM2) 108, a firsttunable optical coupler (TOC) 104, an optical combiner (OC) 105, and anoptical phase shifter (PS) 130. Each of the PM1 106 and the PM2 108 isoperatively coupled to the TOC 104 and the OC 110. The TOC 104 isoperatively coupled to the output of the optical source 102 and has afirst output 111 and a second output 113. The TOC 104 can split (ordivide) the optical signal 110 received from the optical source 102between the first output 111 and the second output 113 to produce afirst split optical signal and a second split optical signal. In someembodiments, the TOC 104 can split the optical signal 110 between thefirst output 111 and the second output 113 at an adjustable (or tunable)power splitting ratio. For example, the TOC 104 can split the opticalsignal 110 such that the first split optical signal at output 111 has anoptical power of 40% of the optical signal 110, and the second splitoptical signal at output 113 has an optical power of 60% of the opticalsignal 110. In other embodiments, the TOC 104 can split the opticalsignal 110 equally such that each of the first split optical signal andthe second split optical signal has a power of 50% of the optical signal110. In some embodiments, the TOC 104 (or an external controller notshown in FIG. 1) can dynamically adjust (or reconfigure) the powersplitting ratio, at any given time when in manufacture or when inoperation, to any ratio between 0 to 100%. In other embodiments, thepower splitting ratio is fixed.

Each of the phase modulators, PM1 106 and PM2 108, can be driven by arespective electrical signal (labeled in FIG. 1 “m-bit” and “n-bit”) andcan modulate an optical signal by varying the instantaneous phase of theoptical signal. Specifically, PM1 106 receives the first split opticalsignal 111 as an input to the PM1 106. PM1 106 then modulates the firstsplit optical signal 111 by applying a first electrical signal (“m-bit”)and outputs a first modulated optical signal 115 to the optical combiner(OC) 105. Similarly, PM2 108 receives the second split optical signal113 as an input to the PM2 108. PM2 108 then modulates the second splitoptical signal 113 by applying a second electrical signal (“n-bit”) tooutput a second modulated optical signal 117 to the OC 105.

In some embodiments, the first electrical signal and the secondelectrical signal applied to the phase modulators, PM1 106 and PM2 108respectively, can be generated by a respective digital-to-analogconverter (DAC, not shown in FIG. 1). A first DAC (not shown in FIG. 1)operatively coupled to PM1 106 can be a 1-bit or multi-bit (or m-bit)DAC (e.g., 2-bit, 4-bit, 8-bit, 16-bit). A second DAC (not shown inFIG. 1) operatively coupled to PM2 108 can also be a 1-bit or multi-bit(or n-bit) DAC (e.g., 2-bit, 4-bit, 8-bit, 16-bit). A multi-bit DAC cangenerate a multi-level electrical signal to be applied to one of the PM1106 and PM2 108. The number of bits of the first DAC can be the same asor different from the number of bits of the second DAC. In someembodiments, the numbers of bits of the first DAC and the second DAC canbe selected (or reconfigured), during a design process, a manufacturingprocess, a reconfiguration process, a troubleshoot process, or inoperation, to adjust (or optimize) coverage on a constellation diagramof the modulated QAM optical signal 120. Details of such embodiments arediscussed below with regards to FIGS. 4-6.

In some embodiments, a phase shifter (PS) 130 can be optionally coupledto PM2 108 and the OC 105. Specifically, PS 130 receives the secondmodulated optical signal 117 from PM2 108 and applies a phase shift orrotation to the second modulated optical signal 117 to produce a thirdmodulated optical signal 140. PS 130, driven by a control signal (notshown in FIG. 1), can cause a phase offset between the first modulatedoptical signal 115 and the third modulated optical signal 140. In someimplementations, the degree of the phase offset can be adjusted (orreconfigured) dynamically to any number between 0 and 90 degrees. Insome implementations as shown in FIG. 1, PS 130 is operatively coupledto the output of PM2 108 and the output of PS 130 is operatively coupledto the input of OC 105. In other implementations, PS 130 can bepositioned before or after each of PM1 106 and PM2 108. For example, thePS can be operatively coupled to the output of the TOC 104 and theoutput of the PS can be operatively coupled to the input of PM2 108. Asdiscussed earlier, PS 130 is optional and thus in some embodiments, theQAM optical modulator 101 does not include a phase shifter 130.

The optical combiner (OC) 105 can be operatively coupled to the output115 of PM1 106 and the output 117 of PM2 108. In the implementationwhere the optional PS 130 is used, the OC 105 can be operatively coupledto the output 115 of PM1 106 and the third modulated optical signal 140and has an output 120. OC 105 can combine the first modulated opticalsignal 115 and the second modulated optical signal 117 (or the thirdmodulated optical signal 140 if the optional PS 130 is used) to output aQAM optical signal 120. In some implementations, the OC 105 can combinethe first modulated optical signal 115 and the second modulated opticalsignal 117 (or the third modulated optical signal 140 if the optional PSis used) at an adjustable (or tunable) power coupling ratio. Forexample, the OC 105 can combine the first modulated optical signal 115and the second modulated optical signal 117 such that the firstmodulated optical signal 115 has an optical power of 60% of the outputQAM optical signal 120, and the second modulated optical signal 117 hasan optical power of 40% of the output QAM optical signal 120. In otherimplementations, the OC 105 can combine the first modulated opticalsignal 115 and the second modulated optical signal 117 such that each ofthe first modulated optical signal 115 and the second modulated opticalsignal 117 has a power ratio of 50% of the output QAM optical signal120. In some implementations, the OC 105 (or an external controller notshown in FIG. 1) can dynamically adjust (or reconfigure) the powercoupling ratio to any ratio between 0 to 100%. In other implementations,the power coupling ratio is fixed. In some implementations, the powercoupling ratio of the OC 105 can be the same as the power splittingratio of the TOC 104. In other implementations, the power coupling ratioof the OC 105 can be different (or unbalanced) from the power splittingratio of the TOC 104.

FIGS. 2A-2B are constellation diagrams of a 16-QAM optical signalgenerated by the QAM optical modulator 101, according to an embodiment.FIG. 2A is a constellation diagram 200 representing a 16-QAM opticalsignal generated by a QAM optical modulator, such as the QAM opticalmodulator 101 discussed above with regards to FIG. 1. As discussed abovein connection with FIG. 1, PM1 106 modulates an optical signal toproduce a first modulated optical signal 115 and PM2 108 modulates anoptical signal to produce a second modulated optical signal 117. The QAMoptical modulator 101 outputs a QAM optical signal 120. When the QAMoptical signal 120 is a 16-QAM optical signal, the 16 constellationpoints can be represented by the black dots (201-208) in the shadedregion 250, as shown in FIG. 2A.

The horizontal axis 230 of the constellation diagram 200 represents anin-phase (I) component of the 16-QAM optical signal, and the verticalaxis 240 of the constellation diagram 200 represents a quadrature (Q)component of the 16-QAM optical signal. In some embodiments, the QAMoptical modulator 101 described with regards to FIG. 1 performs phaseonly modulation. Thus, the first phase modulator, PM1 106 in FIG. 1, cancover any point on circle 210 including the four constellation points(e.g., 202, 203, 204, and 205). The second phase modulator, PM2 108 inFIG. 1, can start from anywhere on circle 210. For example, if PM2 108starts from cross point 209 on circle 210, PM2 108 can cover any pointon circle 220 after a vector addition with the first modulated opticalsignal 115 from PM1 106. As PM2 108 can start from any point on circle210, it can cover all the points within the shaded region 250. In someembodiments, the QAM signals do not occupy the region close to thecenter point in the constellation diagram. In some embodiments, thepower splitting ratio from, for example, TOC 104 in FIG. 1, can beadjusted away from 50:50, to adjust the density of the coverage areainside the shaded region 250 with finite DAC number.

A typical QAM in-phase/quadrature (I/Q) modulator includes an I-arm forin-phase (I) intensity modulation and a Q-arm for quadrature (Q)intensity modulation. When generating a 16-QAM signal using the typicalQAM optical modulator, for example, the I-arm modulator covers fourconstellation points horizontally along the I-axis of the constellationdiagram, and the Q-arm modulator covers four constellation pointsvertically along the Q-axis of the constellation diagram. Each of theI-arm modulator and the Q-arm modulator is typically driven by anindependent four-level electrical signal. Unlike the typical QAMin-phase/quadrature (I/Q) modulator, which performs intensitymodulation, the QAM optical modulator 101 described above with regardsto FIG. 1 performs phase only modulation, in some embodiments. Thus,instead of covering constellation points horizontally along the I-axisby the I-arm modulator and vertically along the Q-axis by the Q-armmodulator of the typical QAM I/Q modulator, the two phase modulators(PM1 106 and PM2 108) of the QAM optical modulator 101 coverconstellation points on circles (e.g., circle 210 and circle 220 in FIG.2A). Modulation loss is greatly reduced with the QAM optical modulator101, compared with the typical QAM I/Q modulator.

Furthermore, the typical QAM I/Q modulator uses only a relatively linearregion of the modulator transfer function, which results innon-uniformity of the constellation points of the constellation diagramand further increases modulation loss. In contrast, the QAM opticalmodulator 101 described above with regards to FIG. 1 can use a broaderregion of the modulator transfer function, including the linear regionthat the typical QAM I/Q modulator uses and the non-linear region. Suchefficient use of the modulator transfer function of the QAM opticalmodulator 101 can further reduce the modulation loss.

FIG. 2B is a constellation diagram of a 16-QAM optical signal generatedby the QAM optical modulator 101, according to an embodiment. In someembodiments, the constellation diagram, as shown in FIG. 2B, can be usedto reversely determine modulation instructions of two phase modulators(e.g., PM1 106 and PM2 108 in FIG. 1) of a QAM optical modulator (e.g.,101 in FIG. 1). Specifically, for example, if a constellation point 282needs to be generated by the QAM optical modulator, a circle 290 coveredby the second phase modulator (e.g., PM2 in FIG. 1) with a center point282 can be determined. Based on the circle 210 covered by a first phasemodulator (e.g., PM1 in FIG. 1), two cross points 284 and 286 can bedetermined. The configuration instructions for the second PM can bedetermined based on a circle with a center point 284 and a circle with acenter point of 286.

FIG. 3 illustrates constellation diagrams of a 16-QAM optical signalbased on simulations of output generated by a QAM optical modulator,according to an embodiment. Each of the six constellation diagrams,310-360, is a simulated constellation diagram of a 16-QAM optical signalgenerated by a configuration of a QAM optical modulator (such as the QAMoptical modulator described with respect to FIG. 1). The horizontal axis(e.g., 301 in diagram 310) of each constellation diagram (e.g., 310)represents an in-phase (I) component of the 16-QAM optical signal, andthe vertical axis (e.g., 302 in diagram 310) of the constellationdiagram (e.g., 310) represents a quadrature (Q) component of the 16-QAMoptical signal. The blue dots (e.g., 311) on each diagram (e.g., 310)represent simulated constellation points covered by the QAM opticalmodulator, and the white area between the dots represents constellationarea not covered by the QAM optical modulator. The red dots (e.g., 391)on each diagram (e.g., 310) represent theoretical constellation pointsof a 16-QAM optical signal. As discussed above with respect to FIG. 1, aset of characteristics of the QAM optical modulator are configurable (orreconfigurable, adjustable, tunable). For example, the power splittingratio (or optical coupling ratio “OCR”) of the tunable optical coupler(such as the TOC 104 in FIG. 1), the power coupling ratio of the opticalcombiner (such as OC 105 in FIG. 1), the number of bits of the DAC thatgenerates the electrical signals that drive phase modulators (such asPM1 and PM2 in FIG. 1), and the phase shift (such as PS 130 in FIG. 1)between a first modulated optical signal (such as 115 in FIG. 1) and asecond modulated optical signal (such as 117 in FIG. 1) arereconfigurable.

As shown in FIG. 3, each of the six constellation diagrams, 310-360,represents a 16-QAM constellation diagram generated by the QAM opticalmodulator with a different set of characteristics of the QAM opticalmodulator. For example, constellation diagram 310 is simulated with theQAM optical modulator having, 305, an optical coupling ratio (“OCR”) of0.5, driven by a DAC with a number of bits of six, and a phase shift ofzero degree. Constellation diagram 320 is simulated with the QAM opticalmodulator having, 306, an OCR of 0.5, driven by a DAC with a number ofbits of six, and a phase shift of fifteen degrees. Constellation diagram330 is simulated with the QAM optical modulator having, 307, an OCR of0.5, driven by a DAC with a number of bits of six, and a phase shift ofthirty degrees. Constellation diagram 340 is simulated with the QAMoptical modulator having, 308, an OCR of 0.4, driven by a DAC with anumber of bits of six, and a phase shift of zero degree. Constellationdiagram 350 is simulated with the QAM optical modulator having, 309, anOCR of 0.3, driven by a DAC with a number of bits of six, and a phaseshift of zero degree. Constellation diagram 360 is simulated with theQAM optical modulator having, 321, an OCR of 0.2, driven by a DAC with anumber of bits of six, and a phase shift of zero degree. As shown inFIG. 3, constellation diagrams 320, 330, and 340 have more coverage ofconstellation points than the other three constellation diagrams 310,350, and 360. Therefore, during a design process, a manufacturingprocess, a reconfiguration process, a troubleshoot process, or inoperation of the QAM optical modulator to produce a 16-QAM opticalsignal, a set of characteristics of the QAM optical modulator can bedynamically chosen to be similar to the characteristics (306, 307, and308) associated with constellation diagrams 320, 330, and 340. In otherwords, by dynamically configuring the OCR, the power coupling ratio ofthe OC, the number of bits of the DAC, and the phase shift of a QAMoptical modulator, at any given time from a design and manufacturingprocess to when the QAM optical modulator is in operation, the coverageof the constellation diagram by the QAM optical modulator can beadjusted or improved.

FIG. 4 illustrates constellation diagrams of a 64-QAM optical signalbased on simulations of output generated by a QAM optical modulator,according to an embodiment. Each of the six constellation diagrams,410-460, is a simulated constellation diagram of a 64-QAM optical signalgenerated by a configuration of a QAM optical modulator (such as the QAMoptical modulator described with respect to FIG. 1). The horizontal axis(e.g., 401 in diagram 410) of each constellation diagram (e.g., 410)represents an in-phase (I) component of the 64-QAM optical signal, andthe vertical axis (e.g., 402 in diagram 410) of the constellationdiagram (e.g., 410) represents a quadrature (Q) component of the 64-QAMoptical signal. Similar to the constellation diagrams of the 16-QAMoptical signal as described with respect to FIG. 3, the blue dots (e.g.,411) on each diagram (e.g., 410) represent simulated constellationpoints covered by the QAM optical modulator, and the white area betweenthe dots represents constellation area not covered by the QAM opticalmodulator. The red dots (e.g., 491) on each diagram (e.g., 410)represent theoretical constellation points of a 64-QAM optical signal.As discussed above with respect to FIG. 1 and FIG. 3, a set ofcharacteristics of the QAM optical modulator are configurable (orreconfigurable, adjustable, tunable) and include the power splittingratio (or optical coupling ratio “OCR”) of the tunable optical coupler(such as the TOC 104 in FIG. 1), the power coupling ratio of the opticalcombiner (such as OC 105 in FIG. 1), the number of bits of the DAC thatgenerates the electrical signals that drive phase modulators (such asPM1 and PM2 in FIG. 1), and the phase shift (such as PS 130 in FIG. 1)between a first modulated optical signal (such as 115 in FIG. 1) and asecond modulated optical signal (such as 117 in FIG. 1) areconfigurable.

As shown in FIG. 4, each of the six constellation diagrams, 410-460,represents a 64-QAM constellation diagram generated by the QAM opticalmodulator with a different set of characteristics of the QAM opticalmodulator. For example, constellation diagram 410 is simulated with theQAM optical modulator having, 405, an optical coupling ratio (“OCR”) of0.5, driven by a DAC with a number of bits of six, and a phase shift ofzero degree. Constellation diagram 420 is simulated with the QAM opticalmodulator having, 406, an OCR of 0.5, driven by a DAC with a number ofbits of six, and a phase shift of fifteen degrees. Constellation diagram430 is simulated with the QAM optical modulator having, 407, an OCR of0.5, driven by a DAC with a number of bits of six, and a phase shift ofthirty degrees. Constellation diagram 440 is simulated with the QAMoptical modulator having, 408, an OCR of 0.4, driven by a DAC with anumber of bits of six, and a phase shift of zero degree. Constellationdiagram 450 is simulated with the QAM optical modulator having, 409, anOCR of 0.4, driven by a DAC with a number of bits of six, and a phaseshift of fifteen degrees. Constellation diagram 460 is simulated withthe QAM optical modulator having, 421, an OCR of 0.3, driven by a DACwith a number of bits of six, and a phase shift of zero degree. As shownin FIG. 4, constellation diagrams 420 and 430 have more coverage ofconstellation points than the other four constellation diagrams 410,440, 450, and 460. Therefore, during a design process, a manufacturingprocess, a reconfiguration process, a troubleshoot process, or inoperation of the QAM optical modulator to produce a 64-QAM opticalsignal, a set of characteristics of the QAM optical modulator can bedynamically chosen to be similar to the characteristics (406 and 407)associated with constellation diagrams 420 and 430. In other words, bydynamically configuring the OCR, the power coupling ratio of the OC, thenumber of bits of the DAC, and the phase shift of a QAM opticalmodulator, at any given time from a design and manufacturing process towhen the QAM optical modulator is in operation, the coverage of theconstellation diagram by the QAM optical modulator can be adjusted orimproved.

FIG. 5 illustrates constellation diagrams of a 256-QAM optical signalbased on simulated output generated by a QAM optical modulator,according to an embodiment. Each of the two constellation diagrams, 510and 520, is a simulated constellation diagram of a 256-QAM opticalsignal generated by a configuration of a QAM optical modulator (such asthe QAM optical modulator described with respect to FIG. 1). Thehorizontal axis, 501, of each constellation diagram represents anin-phase (I) component of the 256-QAM optical signal, and the verticalaxis, 502, of the constellation diagram represents a quadrature (Q)component of the 256-QAM optical signal. Similar to the constellationdiagrams of the 16-QAM optical signal as described with respect to FIG.3 and the constellation diagrams of the 64-QAM optical signal asdescribed with respect to FIG. 4, the shaded blue area (e.g., 511 and521) on each diagram (e.g., 510 and 520) represent simulatedconstellation points and/or area covered by the QAM optical modulator,and the white area between the dots represents constellation area notcovered by the QAM optical modulator. The red dots (e.g., 591) on eachdiagram (e.g., 510) represent theoretical constellation points of a256-QAM optical signal.

As shown in FIG. 5, each of the two constellation diagrams, 510 and 520,represents a 256-QAM constellation diagram generated by the QAM opticalmodulator with a different set of configurable characteristics of theQAM optical modulator. For example, constellation diagram 510 issimulated with the QAM optical modulator having, 505, an opticalcoupling ratio (“OCR”) of 0.5, driven by a DAC with a number of bits ofeight, and a phase shift of zero degree. Constellation diagram 520 issimulated with the QAM optical modulator having, 506, an OCR of 0.435,driven by a DAC with a number of bits of eight, and a phase shift ofzero degree. As shown in FIG. 5, constellation diagram 520 has morecoverage of constellation points 521 than the coverage of constellationpoints 511 of constellation diagram 510. Therefore, during a designprocess, a manufacturing process, a reconfiguration process, atroubleshoot process, or in operation of the QAM optical modulator toproduce a 256-QAM optical signal, a set of characteristics of the QAMoptical modulator can be dynamically chosen to be similar to thecharacteristics, 506, associated with constellation diagrams 520. Inother words, by dynamically configuring the OCR, the power couplingratio of the OC, the number of bits of the DAC, and the phase shift of aQAM optical modulator, at any given time from a design and manufacturingprocess to when the QAM optical modulator is in operation, the coverageof the constellation diagram by the QAM optical modulator can beadjusted or improved.

FIG. 6 is a block diagram illustrating a parallel optical signaltransmission apparatus configured to generate m-QAM optical signals,according to an embodiment. As shown in FIG. 6, the parallel opticalsignal transmission apparatus 600 includes an optical source 602 and aparallel QAM optical modulator 601 operatively coupled to the opticalsource 602. The optical source 602 can be, for example, a laser diodeemitting an optical signal (or light wave) in a continuous waveform(CW), physically and functionally similar to the optical source 102 inFIG. 1. The optical source 602 outputs the optical signal to theparallel QAM optical modulator 601, which can modulate the opticalsignal to output an m-QAM optical signal 620 for transmission in anoptical communication system (not shown in FIG. 6).

In some implementations, the QAM optical modulator 101 in FIG. 1includes two phase modulators (PM1, 106, and PM2, 108), each of which isdriven by a multi-level electrical signal generated by a multi-bit DAC.For example, in one implementation as described in FIG. 3, to generate a16-QAM optical signal, each of the two phase modulators in the QAMoptical modulator (such as the QAM optical modulator 101 in FIG. 1) canbe driven by a DAC with a number of bits of six. Similarly as describedin FIG. 4, in one implementation, to generate a 64-QAM optical signal,each of the two phase modulators in the QAM optical modulator (such asthe QAM optical modulator 101 in FIG. 1) can be driven by a DAC with anumber of bits of six. As described in FIG. 5, in one implementation, togenerate a 256-QAM optical signal, each of the two phase modulators inthe QAM optical modulator (such as the QAM optical modulator 101 inFIG. 1) can be driven by a DAC with a number of bits of eight. The phasemodulators, the TOC, and the OC of the QAM optical modulator 101described with respect to FIG. 1 can be used as building blocks toprovide the parallel QAM optical modulator 601 as described in FIG. 6and the serial QAM optical modulator 701 as described in FIG. 7.

Returning to FIG. 6, in one implementation, the parallel QAM opticalmodulator 601 can include more than two phase modulators operativelycoupled with each other in parallel, such as N phase modulators (PM1621A, PM2 621B . . . PMn 621N). The ellipses 690 represent that therecan be multiple PMs, multiple tunable optical couplers (TOCs), andmultiple optical combiners (OCs). Each phase modulator from the set ofphase modulators (N phase modulators) can be driven by a multi-levelelectrical signal (not shown) generated by a multi-bit DAC (not shown).In some implementations, the number of bits of each DAC can be two orfour, thus generating a binary or four-level electrical signal,respectively. Therefore, to generate a m-QAM optical signal, instead ofhaving only two phase modulators, each of which is driven by anelectrical signal with a higher number of level (e.g., six-level oreight-level electrical signals) as described with respect to FIG. 1, aparallel QAM optical modulator, as described as 601 in FIG. 6, caninclude more than two phase modulators (e.g., N phase modulators)connected in parallel, each of which is driven by an electrical signalwith a lower number of level (e.g., binary or four-level electricalsignal).

Specifically, the parallel QAM optical modulator 601 can include a setof phase modulators (e.g., N phase modulators, N is greater than 3), aset of tunable optical couplers (TOCs), and a set of optical combiners(OCs). In some implementations, when the number of phase modulatorsincluded in the parallel QAM optical modulator 601 N, the number of TOCsincluded in the parallel QAM optical modulator 601 can be (N-1), and thenumber of OCs included in the parallel QAM optical modulator 601 can be(N-1). In one implementation, for example, when N is equal to 3, theparallel QAM optical modulator 601 includes three phase modulators, thefirst phase modulator PM1 621A, the second modulator PM2 621B, and theN^(th) modulator PMn 621N. The parallel QAM optical modulator 601 canalso include a first TOC 603, a second TOC 613, a first OC 604, and asecond OC 614. Optionally, the parallel QAM optical modulator 601 canalso include a set of phase shifters (PS, not shown in FIG. 6) similarto the phase shifter 130 in FIG. 1.

Each of the three phase modulators (PM1 621A, PM2 621B, PMn 621N) can besimilar in structure and function as the phase modulators PM1 and PM2 inFIG. 1. For example, each of the three phase modulators (PM1 621A, PM2621B, PMn 621N) can be driven by a respective electrical signal and canmodulate an optical signal by varying the instantaneous phase of theoptical signal. As previously discussed, in one implementation, theelectrical signal that drives each of the three phase modulators (PM1621A, PM2 621B, PMn 621N) can be a lower-level electrical signal (e.g.,binary or four-level) generated by a DAC (e.g., 2-bit DAC or 4-bit DAC,not shown in FIG. 6). In another implementation, the electrical signalthat drives each of the three phase modulators (PM1 621A, PM2 621B, PMn621N) can be a higher-level electrical signal (e.g., six-level oreight-level) generated by a DAC (e.g., 6-bit DAC or 8-bit DAC, not shownin FIG. 6). The number of bits of the DAC for each PM (PM1 621A, PM2621B, PMn 621N) can be the same as or different from the number of bitsof the DAC for the other PM (PM1 621A, PM2 621B, PMn 621N).

Each of the two TOCs, 603 and 613, can be similar in structure andfunction as the TOC 104 in FIG. 1. Each of the OCs, 604 and 614, can besimilar in structure and function as the OC 105 in FIG. 1. For example apower splitting ratio associated with each of the TOCs, 603 and 613, canbe fixed or dynamically tunable during a design process, a manufacturingprocess, a reconfiguration process, a troubleshoot process, or inoperation. Each of the TOCs, 603 and 613, can split an optical signalequally (i.e., 50%) or unequally (i.e., not 50%). Similarly, a powercoupling ratio associated with each of the OCs, 604 and 614, can befixed or dynamically tunable during a design process, a manufacturingprocess, a reconfiguration process, a troubleshoot process, or inoperation. Each of the OCs, 604 and 614, can combine two optical signalsequally (i.e., 50%) or unequally (i.e., not 50%). As discussed abovewith respect to FIGS. 3-5, the characteristics of each component of theparallel QAM optical modulator 601 are reconfigurable. For example, thepower splitting ratio of each TOCs, 603 and 613, the power couplingratio of each of the OCs, 604 and 614, the number of bits of each DACthat drives each PM (PM1 621A, PM2 621B, PMn 621N), and the phase shiftof each PS (not shown in FIG. 6) are reconfigurable.

In use, the optical source 602 can output an optical signal to the TOC1603 of the parallel QAM optical modulator 601. The TOC1 603 can splitthe optical signal to a first split optical signal and a second splitoptical signal according to a power splitting ratio. The first splitoptical signal is output to the first PM, PM1 621A, and the second splitoptical signal is output to the second TOC, TOC2 613. The first PM, PM1621A, then receives the first split optical signal and modulates thefirst split optical signal to produce a first modulated optical signal,in response to a multi-level electrical signal produced by a multi-bitDAC (not shown in FIG. 6). PM1 621A can output the first modulatedoptical signal to the first OC 604.

Upon receiving the second split optical signal, the second TOC, TOC2613, can split the second split optical signal to produce a third splitoptical signal and a fourth split optical signal according to a powersplitting ratio. TOC2 613 can output the third split optical signal toPM2 621B and output the fourth split optical signal to PMn 621N. PM2621B then receives the third split optical signal and modulates thethird split optical signal to produce a second modulated optical signal,in response to a multi-level electrical signal produced by a multi-bitDAC (not shown in FIG. 6). PM2 621B can output the second modulatedoptical signal to the second OC 614.

In one implementation, PMn 621N then receives the fourth split opticalsignal and modulates the fourth split optical signal to produce a thirdmodulated optical signal, in response to a multi-level electrical signalproduced by a multi-bit DAC (not shown in FIG. 6). PMn 621N can alsooutput the third modulated optical signal to the second OC 614. Uponreceiving the second modulated optical signal and the third modulatedoptical signal, the second OC 614 can combine the second modulatedoptical signal and the third modulated optical signal to produce acombined optical signal according to a power coupling ratio, and outputthe combined optical signal to the first OC 604. Upon receiving thecombined optical signal from the second OC 614 and the first modulatedoptical signal from PM1 621A, the first OC 604 combines the two signals,according to a power coupling ratio, to produce and output a modulatedQAM optical signal 620.

FIG. 7 is a block diagram illustrating a serial optical signaltransmission apparatus configured to generate m-QAM optical signals,according to an embodiment. In some embodiments, similar to the paralleloptical signal transmission apparatus 600 described with respect to FIG.6, the serial optical signal transmission apparatus 700 includes anoptical source 702 and a serial QAM optical modulator 701 operativelycoupled to the optical source 702. The optical source 702 can be, forexample, a laser diode emitting an optical signal (or light wave) in acontinuous waveform (CW), physically and functionally similar to theoptical source 102 in FIG. 1. The optical source 702 outputs the opticalsignal to the serial QAM optical modulator 701, which can modulate theoptical signal to output an m-QAM optical signal 720 for transmission inan optical communication system (not shown in FIG. 7).

Similar to the discussions above with regards to the comparison betweenthe embodiments described in FIG. 6 and the embodiments described inFIG. 1, in some implementations, the serial QAM optical modulator 701can include more than two phase modulators operatively coupled with eachother in serial, such as N phase modulators (PM1 721A, PM2 721B . . .PMn 721N). The ellipses 790 represent that there can be multiple PMs,multiple tunable optical couplers (TOCs), and multiple optical combiners(OCs). Each phase modulator from the set of phase modulators (N phasemodulators) can be driven by a multi-level electrical signal (not shown)generated by a multi-bit DAC (not shown). In some implementations, thenumber of bits of each DAC can be two or four, thus generating a binaryor four-level electrical signal, respectively. Therefore, to generate am-QAM optical signal, instead of having only two phase modulators, eachof which is driven by an electrical signal with a higher number of level(e.g., six-level or eight-level electrical signals) as described withrespect to FIG. 1, a serial QAM optical modulator, as described as 701in FIG. 7, can include more than two phase modulators (e.g., N phasemodulators) connected in serial, each of which is driven by anelectrical signal with a lower number of level (e.g., binary orfour-level electrical signal).

Specifically, the serial QAM optical modulator 701 can include a set ofphase modulators (e.g., N phase modulators, N is greater than 3.), a setof tunable optical couplers (TOCs), and a set of optical combiners(OCs). In some implementations, when the number of phase modulatorsincluded in the parallel QAM optical modulator 601 is N, the number ofTOCs included in the parallel QAM optical modulator 601 can be (N-1),and the number of OCs included in the parallel QAM optical modulator 601can be (N-1). In one implementation, for example, when N is equal to 3,the serial QAM optical modulator 701 includes three phase modulators,the first phase modulator PM1 721A, the second modulator PM2 721B, andthe third modulator PMn, 721N). The serial QAM optical modulator 701 canalso include a first TOC 703, a second TOC 713, a first OC 704, and asecond OC 714. Optionally, the serial QAM optical modulator 701 can alsoinclude a set of phase shifters (PS, not shown in FIG. 7) such as thephase shifter 130 in FIG. 1.

Each of the three phase modulators (PM1 721A, PM2 721B, PMn 721N) can besimilar in structure and function as the phase modulators PM1 and PM2 inFIG. 1. For example, each of the three phase modulators (PM1 721A, PM2721B, PMn 721N) can be driven by a respective electrical signal and canmodulate an optical signal by varying the instantaneous phase of theoptical signal. As previously discussed, in one implementation, theelectrical signal that drives each of the three phase modulators (PM1721A, PM2 721B, PMn 721N) can be a lower-level electrical signal (e.g.,binary or four-level) generated by a DAC (e.g., 2-bit DAC or 4-bit DAC,not shown in FIG. 7). In another implementation, the electrical signalthat drives each of the three phase modulators (PM1 721A, PM2 721B, PMn721N) can be a higher-level electrical signal (e.g., six-level oreight-level) generated by a DAC (e.g., 7-bit DAC or 8-bit DAC, not shownin FIG. 7). The number of bits of the DAC for each PM (PM1 721A, PM2721B, PMn 721N) can be the same as or different from the number of bitsof the DAC for the other PM (PM1 721A, PM2 721B, PMn 721N).

Each of the two TOCs, 703 and 713, can be similar in structure andfunction as the TOC 104 in FIG. 1. Each of the OCs, 704 and 714, can besimilar in structure and function as the OC 105 in FIG. 1. For example apower splitting ratio associated with each of the TOCs, 703 and 713, canbe fixed or dynamically tunable during a design process, a manufacturingprocess, a reconfiguration process, a troubleshoot process, or inoperation. Each of the TOCs, 703 and 713, can split an optical signalequally (i.e., 50%) or unequally (i.e., not 50%). Similarly, a powercoupling ratio associated with each of the OCs, 704 and 714, can befixed or dynamically tunable during a design process, a manufacturingprocess, a reconfiguration process, a troubleshoot process, or inoperation. Each of the OCs, 704 and 714, can combine two optical signalsequally (i.e., 50%) or unequally (i.e., not 50%). As discussed abovewith respect to FIGS. 3-5, the characteristics of each component of theserial QAM optical modulator 701 are reconfigurable. For example, thepower splitting ratio of each TOCs, 703 and 713, the power couplingratio of each of the OCs, 704 and 714, the number of bits of each DACthat drives each PM (PM1 721A, PM2 721B, PMn 721N), and the phase shiftof each PS (not shown in FIG. 7) are reconfigurable.

In use, the optical source 702 can output an optical signal to PM1 721Aof the serial QAM optical modulator 701. Upon receiving the opticalsignal from the optical source 702, PM1 721A can modulate the opticalsignal by applying a multi-level electrical signal produced by amulti-bit DAC (not shown in FIG. 7) to produce a first modulated opticalsignal. PM1 721A can then output the first modulated optical signal toTOC1 703. TOC1 703 can split the first modulated optical signal to afirst split optical signal and a second split optical signal accordingto a power splitting ratio. The first split optical signal is output toPM2 721B, and the second split optical signal is output to the first OC,OC1 704.

Upon receiving the first split optical signal, PM2 721B can thenmodulate the first split optical signal by applying a multi-levelelectrical signal produced by a multi-bit DAC (not shown in FIG. 7) toproduce a second modulated optical signal. PM2 721B can then output thesecond modulated optical signal to TOC2 713. TOC2 713 can split thesecond modulated optical signal to a third split optical signal and afourth split optical signal according to a power splitting ratio. Thethird split optical signal is output to PMn 721N, and the fourth splitoptical signal is output to the second OC, OC2 714.

Upon receiving the third split optical signal, PMn 721N can thenmodulate the third split optical signal by applying a multi-levelelectrical signal produced by a multi-bit DAC (not shown in FIG. 7) toproduce a third modulated optical signal. PM2 721B can then output thethird modulated optical signal to OC2 714. OC2 714 combines the fourthsplit optical signal and the third modulated optical signal, accordingto a power coupling ratio, to produce a first combined optical signal.OC2 714 outputs the first combine optical signal to OC1 704. Uponreceiving the first split optical signal from TOC1 703 and the firstcombined optical signal from OC2 714, OC1 704 can combine the twosignals to produce a modulated QAM optical signal 720.

FIG. 8 is a flow chart illustrating a method of generating a QAM opticalsignal with a QAM optical modulator having two phase modulators,according to an embodiment. This method 800 can be implemented at a QAMoptical modulator (e.g., QAM optical modulator 101 in FIG. 1). The QAMoptical modulator, which is operatively coupled to an optical source,includes a tunable optical coupler (TOC), a first phase modulator (PM) asecond phase modulator (PM), and an optical combiner (OC). The QAMoptical modulator can optically include a phase shifter (PS) in someimplementations. The method includes receiving an optical signal at theTOC of the QAM optical modulator from the optical source at 802. The TOCthen splits the optical signal, according to a first power splittingratio, to produce a first split optical signal and a second splitoptical signal. In some implementations, the TOC can split the opticalsignal equally, and thus the first power splitting ratio is 0.5. Inother implementations, the TOC can split the optical signal unequally.The first power splitting ratio can also be tunable or fixed.

At 804, the TOC outputs the first split optical signal to the first PMand the second split optical signal to the second PM. Upon receiving thefirst split optical signal, the first PM modulates the first splitoptical signal by applying a multi-level electrical signal generated bya first multi-bit DAC and produces a first modulated optical signal. Thefirst PM then outputs the first modulated optical signal to the OC. Theinstantaneous phase of the first split optical signal is varied when thefirst PM is driven by the first multi-level electrical signal. The firstmodulated optical signal can be represented as covering a first set ofconstellation points on a circle in a constellation diagram of the QAMoptical signal.

At 806, upon receiving the second split optical signal, the second PMmodulates the second split optical signal by applying a multi-levelelectrical signal generated by a second multi-bit DAC and produces asecond modulated optical signal. The second PM then outputs the secondmodulated optical signal to the OC. The instantaneous phase of thesecond split optical signal is varied when the second PM is driven bythe second multi-level electrical signal. The number of bits of thefirst DAC can be the same as or different from the number of bits of thesecond DAC. For example, the first DAC, which is operatively coupled tothe first PM, can have a number of bits of six to generate a 16-QAMoptical signal. The second DAC, which is operatively coupled to thesecond PM, can have a number of bits of six or eight. The secondmodulated optical signal can be represented within the constellationdiagram as starting from a point on the circle covered by the firstmodulated optical signal and covering a second set of constellationpoints. The first set of constellation points and the second set ofconstellation points represent all constellation points of a QAM opticalsignal.

In some embodiments, a phase shifter (PS) can be optionally coupled tosecond PM and the OC. Specifically, the PS receives the second modulatedoptical signal from the second PM and applies a phase rotation to thesecond modulated optical signal to produce a third modulated opticalsignal. PS, driven by a control signal, can cause a phase offset betweenthe first modulated optical signal and the third modulated opticalsignal. In some implementations, the degree of the phase offset can bedynamically adjusted (or reconfigured) to any number between 0 and 90degrees. In some implementations as shown in FIG. 1, the PS isoperatively coupled to the output of second PM and the output of PS isoperatively coupled to the input of OC. In other implementations, the PScan be positioned before or after each of the first PM and the secondPM. For example, the PS can be operatively coupled to the output of theTOC and the output of the PS can be operatively coupled to the input ofsecond PM. As discussed earlier, PS is optional and thus in someembodiments, the QAM optical modulator does not include a phase shifter.

At 808, when receiving the first modulated optical signal from the firstPM and the second modulated optical signal from the second PM, the OCcombines the first modulated optical signal and the second modulatedoptical signal, according to a power coupling ratio, to output a QAMoptical signal. In some implementations, the OC can combine the firstmodulated optical signal and the second modulated optical signalequally, and thus the power coupling ratio is 0.5. In otherimplementations, the OC can combine the first modulated optical signaland the second modulated optical signal unequally. The power couplingratio can also be dynamically tunable or fixed. In some implementations,the power coupling ratio of the OC can be the same as the powersplitting ratio of the TOC. In other implementations, the power couplingratio of the OC can be different (or unbalanced) from the powersplitting ratio of the TOC. Therefore, during a design process, amanufacturing process, a reconfiguration process, a troubleshootprocess, or in operation of the QAM optical modulator to produce a QAMoptical signal, a set of characteristics of the QAM optical modulatorcan be dynamically chosen to be similar to the characteristicsassociated with simulated constellation diagrams that have highercoverage by the constellation points. In other words, by dynamicallyconfiguring the OCR, the number of bits of the DAC, the power couplingratio of the OC, and the phase shift of a QAM optical modulator, at anygiven time from a design and manufacturing process to when the QAMoptical modulator is in operation, the coverage of the constellationdiagram by the QAM optical modulator can be adjusted or improved.

In some implementations, a set of characteristics of the QAM opticalmodulator are configurable (or reconfigurable, adjustable, tunable). Forexample, the power splitting ratio (or optical coupling ratio “OCR”) ofthe TOC, the power coupling ratio of the optical combiner, the number ofbits of the DAC which generates the electrical signals that drive phasemodulators, and the phase shift are reconfigurable.

FIG. 9 is a flow chart illustrating a method of generating a QAM signalwith a parallel or serial QAM optical modulator having three or morephase modulators, according to an embodiment. The method 900 can beimplemented in a parallel QAM optical modulator (e.g., parallel QAMoptical modulator 601 in FIG. 6) or a serial QAM optical modulator(e.g., serial QAM optical modulator 701 in FIG. 7). The parallel orserial QAM optical modulator, which is operatively coupled to an opticalsource, includes three or more phase modulators, two or more TOCs, twoor more OCs, and optional phase shifters.

At 902, the parallel or serial QAM optical modulator receives an opticalsignal from the optical source and splits the optical signal to producea set of split optical signals. At 904, each PM from the set of PMsmodulates a split optical signal by applying a multi-level electricalsignal to produce a set of modulated optical signals. At 906, the set ofOCs combines the set of modulated optical signals to output a modulatedQAM optical signal. In some implementations, the number of bits of eachDAC can be two or four, thus generating a binary or four-levelelectrical signal, respectively. Therefore, to generate a m-QAM opticalsignal, the parallel or serial QAM optical modulator, as described as601 in FIGS. 6 and 701 in FIG. 7, can include three or more phasemodulators (e.g., N phase modulators) connected in parallel or serial,each of which is driven by an electrical signal with a lower number oflevel (e.g., binary or four-level electrical signal).

Some embodiments described herein relate to a system including aprocessor. The processor can include one or more modules configured toperform different functions associated with generating a QAM opticalsignal, including, but not limited to, performing dynamic adjustment ofpower splitting ratios of tunable optical couplers, power couplingratios of optical combiners, a number of bits of DACs, and a phase shiftof a QAM optical modulator. In some embodiments, the processor cangenerate and/or transmit control signals and/or modulation signals. Insome embodiments, the control signals can be associated with a phaseshifter (PS) to adjust the degree of the phase shift between two opticalsignals. In some embodiments, the control signals can be associated withconfiguring for a particular M-ary value. In some embodiments, themodule(s) included in the processor can be a hardware-based module(e.g., an ASIC, a DSP, a FPGA), a software-based module (e.g., a moduleof computer code executed at a processor, a set of processor-readableinstructions executed at a processor), and/or a combination of hardware-and software-based modules. Some embodiments described herein relate toa computer storage product with a non-transitory computer-readablemedium (also can be referred to as a non-transitory processor-readablemedium) having instructions or computer code thereon for performingvarious computer-implemented operations. The computer-readable medium(or processor-readable medium) is non-transitory in the sense that itdoes not include transitory propagating signals per se (e.g., apropagating electromagnetic wave carrying information on a transmissionmedium such as space or a cable). The media and computer code (also canbe referred to as code) may be those designed and constructed for thespecific purpose or purposes. Examples of non-transitorycomputer-readable media include, but are not limited to: magneticstorage media such as hard disks, floppy disks, and magnetic tape;optical storage media such as Compact Disc/Digital Video Discs(CD/DVDs), Compact Disc-Read Only Memories (CD-ROMs), and holographicdevices; magneto-optical storage media such as optical disks; carrierwave signal processing modules; and hardware devices that are speciallyconfigured to store and execute program code, such asApplication-Specific Integrated Circuits (ASICs), Programmable LogicDevices (PLDs), Read-Only Memory (ROM) and Random-Access Memory (RAM)devices. Other embodiments described herein relate to a computer programproduct, which can include, for example, the instructions and/orcomputer code discussed herein.

Examples of computer code include, but are not limited to, micro-code ormicroinstructions, machine instructions, such as produced by a compiler,code used to produce a web service, and files containing higher-levelinstructions that are executed by a computer using an interpreter. Forexample, embodiments may be implemented using imperative programminglanguages (e.g., C, Fortran, etc.), functional programming languages(Haskell, Erlang, etc.), logical programming languages (e.g., Prolog),object-oriented programming languages (e.g., Java, C++, etc.) or othersuitable programming languages and/or development tools. Additionalexamples of computer code include, but are not limited to, controlsignals, encrypted code, and compressed code.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Where methods described above indicate certain eventsoccurring in certain order, the ordering of certain events may bemodified. Additionally, certain of the events may be performedconcurrently in a parallel process when possible, as well as performedsequentially as described above.

What is claimed is:
 1. An apparatus, comprising: a serial quadratureamplitude modulation (QAM) optical modulator including a first phasemodulator (PM), a first tunable optical coupler (TOC), a second PM, asecond TOC, a third PM, a first optical combiner (OC), and a secondoptical combiner (OC), the first PM, operatively coupled to an input ofthe first TOC, configured to modulate an input optical signal inresponse to a first multi-level electrical signal to produce a firstmodulated optical signal, the first TOC having a first outputoperatively coupled to the second PM and a second output operativelycoupled to the first OC, the second PM operatively coupled to an inputof the second TOC, the second TOC having a first output operativelycoupled to the third PM and a second output operatively coupled to thesecond OC, the third PM operatively coupled to an input of the secondOC, an output of the second OC coupled to a first input of the first OC,the first OC configured to output a modulated QAM optical signal.
 2. Theapparatus of claim 1, wherein: the first PM is configured to modulatethe input optical signal to produce a first modulated optical signal,the first TOC is configured to split the first modulated optical signalto produce a first split optical signal and a second split opticalsignal, the second PM is configured to modulate the first split opticalsignal to produce a second modulated optical signal, the second TOC isconfigured to split the second modulated optical signal to produce athird split optical signal and a fourth split optical signal, the thirdPM is configured to modulate the third split optical signal to produce athird modulated optical signal, the second OC is configured to combinethe third modulated optical signal and the fourth split optical signalto produce a first combined optical signal, and the first OC isconfigured to combine the first combined optical signal with the secondsplit optical signal to produce the modulated QAM optical signal.
 3. Anapparatus, comprising: a serial quadrature amplitude modulation (QAM)optical modulator including a first phase modulator (PM), a firsttunable optical coupler (TOC), a second PM, a second TOC, a third PM, afirst optical combiner (OC), and a second optical combiner (OC), thefirst PM, operatively coupled to the first TOC, configured to modulatean input optical signal in response to a first multi-level electricalsignal to produce a first modulated optical signal, the first TOCconfigured to split the first modulated optical signal to produce afirst optical signal and a second optical signal, the second PMconfigured to modulate the first split optical signal in response to asecond multi-level electrical signal to produce a second modulatedoptical signal, the second TOC configured to split the second modulatedoptical signal to produce a third split optical signal and a fourthsplit optical signal, the third PM configured to modulate the thirdsplit optical signal in response to a third multi-level electricalsignal to produce a third modulated optical signal, the second OCconfigured to combine the third modulated optical signal and the fourthsplit optical signal to produce a first combined optical signal, thefirst OC configured to combine the first combined optical signal and thesecond split optical signal to generate a QAM optical signal.
 4. Theapparatus of claim 3, wherein: the serial QAM optical modulator includesa plurality of phase modulators connected in series.
 5. The apparatus ofclaim 3, wherein: the serial QAM optical modulator includes a set ofphase shifters configured to shift a phase of one or more of the firstmodulated optical signal, the second modulated optical signal, or thethird modulated optical signal.
 6. The apparatus of claim 5, wherein thephase of one or more of the first modulated optical signal, the secondmodulated optical signal, or the third modulated optical signal shiftedby the set of phase shifters is configurable.
 7. The apparatus of claim3, wherein: the first TOC is configured to split the first modulatedoptical signal at an adjustable power splitting ratio to produce thefirst optical signal and the second optical signal.
 8. The apparatus ofclaim 3, wherein: the first OC is configured to combine the firstcombined optical signal and the second split optical signal at anadjustable coupling ratio.
 9. The apparatus of claim 3, wherein: thefirst PM is configured to modulate the input optical signal to producethe first modulated optical signal such that the first modulated opticalsignal is represented within a constellation diagram as covering a firstplurality of constellation points on a circle, the second PM isconfigured to modulate the first split optical signal to produce thesecond modulated optical signal such that the second modulated opticalsignal is represented within the constellation diagram as starting froma point on the circle and covering a second plurality of constellationpoints, the third PM is configured to modulate the third split opticalsignal to produce the third modulated optical signal such that the thirdmodulated optical signal is represented within the constellation diagramas starting from one of the second plurality of constellation points,and covering a third plurality of constellation points, and a vectoraddition of the first plurality of constellation points, the secondplurality of constellation points and the third plurality ofconstellation points collectively represents the QAM optical signal. 10.The apparatus of claim 3, wherein the QAM optical signal is one of a16-QAM optical signal, a 64-QAM optical signal, or a 256-QAM opticalsignal.
 11. The apparatus of claim 3, wherein a number of bits of thesecond multi-level electrical signal equals a number of bits of thefirst multi-level electrical signal.
 12. The apparatus of claim 3,wherein a number of bits of the second multi-level electrical signaldiffers from a number of bits of the first multi-level electricalsignal.
 13. The apparatus of claim 3, wherein the serial QAM opticalmodulator is configured to be operatively coupled to a digital-to-analogconverter that generates the first multi-level electrical signal. 14.The apparatus of claim 13, wherein a number of bits associated with thedigital-to-analog converter is configurable.
 15. A method, comprising:modulating an input optical signal in response to a first multi-levelelectrical signal to produce a first modulated optical signal; splittingthe first modulated optical signal to produce a first split opticalsignal and a second split optical signal; modulating the first splitoptical signal in response to a second multi-level electrical signal toproduce a second modulated optical signal; splitting the secondmodulated optical signal to produce a third split optical signal and afourth split optical signal; modulating the third split optical signalin response to a third multi-level electrical signal to produce a thirdmodulated optical signal; combining the third modulated optical signaland the fourth split optical signal to produce a first combined opticalsignal; and combining the first combined optical signal and the secondsplit optical signal to generate a QAM optical signal.
 16. The method ofclaim 15, wherein: the modulating the input optical signal, themodulating the first split optical signal, and the modulating the thirdsplit optical signal are performed in a serial order.
 17. The method ofclaim 15, further comprising: shifting a phase of one or more of thefirst modulated optical signal, the second modulated optical signal, orthe third modulated optical signal.
 18. The method of claim 17, whereinthe phase of one or more of the first modulated optical signal, thesecond modulated optical signal, or the third modulated optical signalis configurable.
 19. The method of claim 15, further comprising:splitting the first modulated optical signal at an adjustable powersplitting ratio to produce the first split optical signal and the secondsplit optical signal.
 20. The method of claim 15, further comprising:combining the first combined optical signal and the second split opticalsignal at an adjustable coupling ratio.